1. Field of the Invention
The present invention relates to a superconducting cyclotron of the type which does not include an iron yoke and separately to target apparatus and a method for producing isotopes such as technetium-99m utilising the ionised particles produced by a particle accelerator.
2. Description of the Related Technology
A yoke-less superconducting cyclotron, i.e. one which does not include an iron yoke, is described in EP 0221987 and is manufactured by Oxford Instruments under the name OSCAR*. The cyclotron consists of a superconducting magnet having a set of coils housed in a cryostat. The cryostat surrounds an inner chamber within which is located two sets of iron pole pieces, one above the other, between which is provided a beam space in which particles are accelerated. The pole pieces are arranged to interact with the magnetic field generated by the superconducting magnet to render the magnetic field isochronous with an azimuthal variation in strength. The particles are accelerated within the beam space by a large oscillating voltage which is applied across the beam space. FNT * Trade Mark
The development of such known yoke-less superconducting cyclotrons has significantly reduced the general bulk and weight of cyclotrons. However, such "compact" accelerators are limited in the maximum current of the particle beam which can be delivered. For example, the OSCAR* cyclotron is considered to have a theoretical maximum beam current limit of 500 .mu.A for 12 MeV particles (the proton energy best suited for neutron radiography or as a PET isotope producer). One of the significant factors which limits any increase in beam current is the phenomenon known as space charge FNT * Trade Mark blow-up. This phenomenon is the tendency of like charged particles in a beam of ions to repel one another. Since this results in the axial dimension of the beam increasing, at higher currents the circulating particles in the beam have a tendency to strike the RF accelerating structures in the cyclotron. This phenomenon is most noticeable during the first few orbits of the particles in the beam space.
Even if higher beam currents could be obtained from such known cyclotrons, there is a further problem that at higher beam currents, where the ionised particles are to be used in direct production of technetium-99m by means of bombardment of an external target with the accelerated particles, it is difficult to extract the beam from the cyclotron and supply the beam to the target enclosure without causing contamination of the cyclotron with radioactivity from the target bombardment process. Use of a cooled thin exit window to screen the cyclotron from the radioactivity is only effective for beam currents up to approximately 200 .mu.A.
In a conventional cyclotron target it is common for target material to be deposited onto a water cooled copper backing to provide a mechanism for conducting heat away from the target during bombardment by high energy charged particles, but this requires the isotopes produced to be extracted from the target by destructive chemical processing which prevents the target from being reirradiated. This method is therefore undesirable for .sup.99m Tc production which involves many short irradiations due to the short half life and requires the use of expensive enriched target material which must be frequently recycled. Also, the production of .sup.99m Tc on a commercial scale, for example for regional distribution to hospitals, requires the use of high power cyclotrons such as is described herein which have the capacity to deliver milliamperes of proton beam current to a target at energies up to 30 MeV. For example, in order to produce about 300 Ci/day of .sup.99m Tc at end of bombardment a 3 mA beam current is required and the target assembly must be capable of withstanding up to 90 kW power.
Also, at high beam currents, to date difficulties have been encountered in extracting .sup.99m Tc on-line from the expensive .sup.100 Mo target and ensuring the target material is retained or returned in a suitable target form.